THE INHERITANCE OF FAMILY TRAITS 115 



should avoid having children, while non-affected may nnarry 

 if the disease first appeared in the grandparents at 50 or 

 after. If it appeared earlier it would seem to be prudent 

 for the normal persons to delay reproduction until within 

 ten years of the time that the defect appeared in their parents. 

 Then if no trace of the disease has occurred they may have 

 children with impunity. 



h. Megalophthalmus or protruding eye. A rather rare 

 disease of whose inheritance there can be no doubt, although 

 the exact method of that inheritance is uncertain. Persons 

 with a well marked case had best avoid reproduction. 



i. Nystagmus, or "swimming eyes." This is due to 

 spasmodic contractions of the eye muscles and may or may 

 not be associated with other defects of the eye. The dis- 

 orders with which it is most apt to be associated are : strabis- 

 mus, retinitis pigmentosa, coloboma, albinism, microphthal- 

 mus and cataract. 



In some of the pedigrees that have been pubHshed 

 (Clarke's, 1903), nystagmus, Uke optic nerve atrophy, is not 

 expressed in the (simplex) females ' but is expressed in all 

 males capable of transmitting it. When it is unexpressed 

 in the males of the strain, it will probably not (in non- 

 consanguineous marriages) appear in the offspring. But mar- 

 riages of even non-affected females (unless from large fami- 

 lies of non-affected brothers) and of all affected males are 

 pretty certain to yield offspring with nystagmus. 



k. Paralysis or imperfect development of the muscles of eye 

 and lids. — This includes ptosis, or drop of the upper eye- 

 hd; epicanthus, a fold of skin passing from nose to eyebrow 

 over the inner corner of the eye ; blepharophimosis, or small- 

 ness of opening of eyelids; ophthalmoplegia, or paralysis of 

 eye muscles; strabismus or squinting. Every one of these 

 peculiarities shows clear evidence of heredity. 



1 In other families nystagmus appears also in the females. 



